Baseline

The first component for any corridor is a baseline. The baseline is composed of two Autodesk^® AutoCAD^® Civil 3D^® objects, an alignment providing the horizontal layout, and a profile providing the vertical layout. The baseline generates the backbone on which the assembly is placed. Corridors can contain more than one baseline. Therefore, a network of roads can be modeled using one corridor.

In most of the examples in this chapter, the baseline will correspond to the centerline alignment with a profile representing the elevation at the crown of a proposed road. However, this is not always the case, as we will explore in more depth in Chapter 10, “Advanced Corridors, Intersections, and Roundabouts.” As your designs become more detailed, you may have corridors with multiple baselines.

Region

When the geometry along a baseline changes enough to warrant a new assembly, a new region is needed. Regions specify the station range where a specific assembly is applied to the design. There may be many regions along a baseline to accommodate design geometry, but the regions may not overlap. Therefore, each region has a start station and an end station, with the end station of one region often matching the start station of the next region.

Assembly

Marker points, links, and shapes are coded into the subassemblies that the assembly contains, as you saw in Chapter 8, “Assemblies and Subassemblies.” Assemblies are the third Civil 3D object that is required to generate the corridor by providing cross-sectional information to be applied along some or all of the length of the baseline. Figure 9.4 shows a symmetric roadway assembly that consists of lanes, curbs, sidewalks, and daylight links.

Frequency

Frequency refers to how often the assembly is applied to the corridor design. You can set the frequency for the corridor as a whole or for each region.

The frequency value will vary depending on the situation. The default frequencies of the stock Civil 3D templates are 25' in Imperial units and 20 m for metric units. But like many of the settings in Civil 3D, these defaults can be changed in your drawing or template settings by using the following simple steps:

1. On the Settings tab of Toolspace, expand the Corridor &cmdarr; Commands branch.
2. Right-click the Create Corridor command and choose Edit Command Settings to display the Edit Command Settings – Create Corridor dialog.
3. Expand Assembly Insertion Defaults.
4. Change Frequency Along Tangents, Frequency Along Curves, Frequency Along Spirals, Frequency Along Profile Curves, or any other of the multitude of default settings associated with creating a corridor.

A new feature in Civil 3D 2016 is an alternative to configuring frequency along curvatures. Instead of using a constant distance value for frequency, frequency can be triggered by the mid-ordinate distant value. This frequency setting can be applied to curves in the alignment or curves in offset targets (targets will be discussed in the next section), resulting in more frequent insertions of assemblies along smaller curves and less frequent insertions of assemblies along larger curves. This produces a more precise and accurate 3D model without a lot of manual overrides on the region level.

In addition to the frequencies based on the alignment or profile entity, Civil 3D can place frequency lines at special stations such as horizontal geometry stations, superelevation critical stations, profile geometry stations, profile high/low stations, and offset target geometry stations. You can also manually create additional frequency stations for things such as driveways or culvert crossings.

Target

As you learned in Chapter 8, three types of targets can be configured in a corridor: a target surface, a target elevation, and a target offset. Targets can be used in lieu of additional assemblies to change corridor geometric characteristics such as cross slope (elevation targets) and lane width (offset target). Surface targets can be used in daylighting or roadway rehabilitation scenarios. Some targets are optional, whereas others are required. For a detailed explanation on targeting for each stock assembly, refer to the help files.

Corridor Feature Lines

When a corridor is created, corridor feature lines are generated. These feature lines can represent back of curb, top of curb, flow lines, edges of pavement, crowns, and any other breakline that would be produced based on your proposed typical roadway section. Corridor feature lines are drawn along the corridor, connecting marker points of identical codes in between assembly frequencies, as shown in Figure 9.5. This takes traditional roadway design to another level: A collection of cross sections occurring at specified frequencies becomes a dynamic three-dimensional object model.

Later in this chapter, you will take a closer look at corridor feature lines.

This exercise gives you hands-on experience in building a corridor model from an alignment, a profile, and an assembly:

1. Open the 0901_RoadCorridor.dwg file or 0901_RoadCorridor_METRIC.dwg file. You can download these files from www.sybex.com/go/masteringcivil3d2016. Note that the drawing has several alignments, profile views containing the existing and design profiles for each, an assembly, and an existing ground surface.
2. From the Home tab Create Design panel, choose Corridor to display the Create Corridor dialog.
3. In the Name text box, name the corridor North River Crossing.

   Keep the default values for Corridor Style and Corridor Layer.

4. Set Alignment to ROAD C and set Profile to FG-ROAD C (take extra care not to select EG-ROAD C by mistake).

   Notice that you can use the small green selection button to select the object on the screen instead of using the drop-down list if you prefer.

5. Verify that Assembly is set to Urban 14' Single-Lane (or Urban 4.5m Single-Lane for metric users).
6. Verify that Target Surface is set to Existing Surface.
7. Verify that the Set Baseline And Region Parameters check box is selected.

   The Create Corridor dialog should now look similar to Figure 9.6.

   

8. Click OK to accept the settings and to display the Baseline And Region Parameters dialog, as shown in Figure 9.7.

   

   Notice that there is currently one baseline containing one region. The Start Station and End Station values of the region match those of the baseline.

   Most of the other settings have already been set from the information you provided in the Create Corridor dialog, but let's take a few minutes to look at some of the settings in the Baseline And Region Parameters dialog.

9. Click the ellipsis button in the Frequency column in the first row associated with the baseline (BL) to display the Frequency To Apply Assemblies dialog.

   By clicking the ellipsis in the first row that is associated with the baseline, you will be setting the frequency for all of the regions within that baseline. In this instance you have only one region, but setting them all at once is a good habit to get into when applicable. You could also click the Set All Frequencies button, which would set the frequency for all the baselines and all the baseline regions.

10. Examine the settings in the Frequency To Apply Assemblies dialog, as shown in Figure 9.8.

    

    You can vary the default frequency distance for the portions of the region along tangents, curves, spirals, and profile curves. You can also base frequency along alignment and offset curves by curvature (mid-ordinate distance) instead of by increment. In addition, you can add frequency lines at various geometry points. At the bottom of the dialog you can add a user-defined station.

11. Click OK to accept the settings in the Frequency To Apply Assemblies dialog.
12. Click the ellipsis button in the Targets column in the baseline row to display the Target Mapping dialog.

    The ellipsis buttons in this column behave the same way as the ellipsis buttons in the Frequency column. Notice that there is also a Set All Targets button, which can be used to set the targets for all of the baselines and all of the baseline regions.

13. Examine the settings in the Target Mapping dialog, as shown in Figure 9.9.

    

    The information in this dialog will vary depending on the subassemblies assigned to the assembly used for this corridor baseline but will always be broken into three categories: Surfaces, Width Or Offset Targets, and Slope Or Elevation Targets. Notice that because you selected Existing Surface as the target surface when creating the corridor, Existing Surface is already set as the target surface for the left and right subassemblies targeting a surface.

14. Click OK to accept the settings in the Target Mapping dialog.
15. Click OK to accept the settings in the Baseline And Region Parameters dialog.

    If at any point you want to return to the information shown in this dialog, you can do so on the Parameters tab of the Corridor Properties dialog, which you will look at a little later in this chapter.

    You will see a dialog warning you that the corridor definition has been modified and giving you two options: Rebuild The Corridor or Mark The Corridor As Out-Of-Date. Select the Rebuild The Corridor option.

    If you click the check box at the bottom of this warning dialog that says “Always perform my current choice,” you will not see this warning dialog again.

    You will receive an error message in Panorama, as shown in Figure 9.10, that reads “Intersection with target could not be computed,” “Intersection Point doesn't exist,” or something similar. You will rectify this issue in the following steps.

    

    If Panorama did not automatically display, from the Home tab Palettes expanded panel, choose Event Viewer.
    

16. Dismiss Panorama.

    Your corridor should look similar to Figure 9.11.

    

    Now, let's try to figure out why those messages appeared in Panorama. In plan view, at the last station on the east end of the alignment, notice that on the left side some of the feature lines are not connecting (shown in Figure 9.11). Daylighting could not occur at the final station because the alignment is extending outside the outer edge of the surface. Furthermore, a look at the corridor in the Object Viewer shows a “waterfall” (see Figure 9.12).

    

    The fact that the alignment is extending outside the outer edge of the surface isn't the only cause of the error in Panorama. If you take a look at the profile view for ROAD C, the FG-ROAD C profile does not extend to the end of the alignment, which explains the waterfall. A waterfall occurs when the profile used in the corridor is not providing elevation data along a station range of the alignment. Therefore, the corridor dives down to elevation zero in that station range. In this case, you need to shorten the corridor so that it builds along the station range matching FG-ROAD C.

17. Select the corridor to activate the Corridor contextual tab.
18. From the Corridor contextual tab Modify Corridor panel, choose the Corridor Properties icon to display the Corridor Properties dialog.
    
19. In the Corridor Properties dialog, switch to the Parameters tab.

    Notice in Figure 9.13 that the end station for the region is 20+43.16' (or 0+622.76m for metric users), which is based on the full length of the alignment. However, the design profile ends at 19+95' (or 0+608m for metric users). Even though there is no elevation assigned at 20+43.16' (or 0+622.76m for metric users), the corridor assumes an elevation of zero at this station, which creates this waterfall effect. To fix this, you need to tell the corridor to end the region at the final station of the design profile.

    Note that even though the display value shows only two (or three) decimal places, the corridor examines up to eight decimal places of precision, which means that the value being displayed as the final station of the design profile may be a rounded-up value. If this is the case, this too will create a waterfall effect.

20. Click into the End Station field for the region and enter 1995 (or 608 for metric users).

    Entering the value with station notation (the plus sign) is not needed.

    Notice that you could alternatively click the station picker button in this cell to select the station on the plan.

21. Click OK to accept the settings in the Corridor Properties dialog.
22. If you didn't click the check box at the last warning dialog, you will receive the same warning again. If so, select the Rebuild The Corridor option to allow the corridor to rebuild.

    No new errors will appear in Panorama.

A quick look in the Object Viewer should reveal that the waterfall is gone. When this exercise is complete, you can close the drawing. A finished copy of this drawing is available from the book's website with the filename 0901_RoadCorridor_FINISHED.dwg or 0901_RoadCorridor_METRIC_FINISHED.dwg.

Rebuilding Your Corridor

A corridor is a dynamic model—which means that if you modify any of the objects used to create the corridor, the corridor must be rebuilt to reflect those changes. For example, if you make a change to the design profile, you must rebuild the corridor to bring it up to date. The same principle applies to changes to alignments, assemblies, target surfaces, and any other corridor building blocks or parameters.

You can access the Rebuild command by right-clicking the corridor name in Prospector, as shown in Figure 9.14.

You can also rebuild the corridor or rebuild all corridors by selecting the corridor object and choosing Rebuild Corridor or Rebuild All Corridors from the Corridor contextual tab Modify Corridor panel, as shown in Figure 9.15.

Tweaking Corridors

Your corridor may not be perfect on your first iteration of the design. Become comfortable getting into and working with the Parameters tab of the Corridor Properties dialog. You will spend the majority of your time modifying your corridor on this tab.

Whether you are building your first corridor or your five hundredth, odds are good that you will run into one of the following common issues:

1. Problem The waterfall effect—your corridor seems to fall off a cliff, meaning the beginning or ending station of your corridor drops down to elevation zero.
   1. Typical Cause The range of your corridor is longer than the design profile.
   2. Fix This is exactly what you ran into in the first exercise. The corridor takes the initial station range from your alignment. However, sometimes you don't tie into existing ground at the exact alignment start and end stations, so you need to adjust the corridor stations accordingly.

      If you need to check the exact start and end stations of the design profile, the best place to do so is the Profile Data tab in the Profile Properties dialog (as shown in Figure 9.16). Edit your corridor region to begin and end at the design profile station.

      

      Alternatively, you can use the station picker button to select the first and last corridor region stations by snapping to the first and last frequency lines that correspond to your profile geometry.

2. Problem Your corridor seems to take longer to build and has irregular frequency stations. Also, your daylighting may not extend out to where you expect it (see Figure 9.17).

   

   1. Typical Cause You accidentally chose the existing profile instead of the design profile for your baseline profile. Most corridors are set up to place a frequency line at every vertical geometry point, and a profile, such as the existing profile, has many more vertical geometry points than a layout profile (in this case, the design profile). These additional points on the existing profile are the cause of the unexpected frequency lines.
   2. Fix Always use care to choose the correct profile. Either physically select the profile onscreen or make sure your naming conventions clearly define your finished grade as finished grade. If your corridor is already built, select the corridor, right-click, and choose Corridor Properties, as shown in Figure 9.18. On the Parameters tab of the Corridor Properties dialog, change the profile from the existing profile to the design profile.

      

Adding a surface target throws another variable into the mix. Here is a list of some of the most typical problems new users face and how to solve them:

1. Problem Your corridor doesn't show daylighting even though you have a daylight subassembly on your assembly. You may also get an error message in Event Viewer, as shown in Figure 9.19.

   

   1. Typical Cause You forgot to set the surface target when you created your corridor.
   2. Fix Select the corridor, right-click, and choose Corridor Properties. On the Parameters tab of the Corridor Properties dialog, click the Set All Targets button. The Target Mapping dialog opens, and its first category is Surfaces. Click the <Click Here To Set All> text in the Object Name column field to display the Pick A Surface dialog. In this dialog, you can choose a surface for the daylight subassembly to target.
2. Problem Your corridor seems to be missing areas of daylighting. You may also get an error message in Event Viewer, as shown in Figure 9.20.

   

   1. Typical Cause The Daylight link cannot find the target surface within the parameters you've set in the subassembly properties. This could also mean that the target surface doesn't fully extend the full length of your corridor or your target surface is too narrow at certain locations.
   2. Fix Add more data to your target surface. The daylighting subassemblies in your corridor need a surface to tie into at the slope, grade, or distance specified in subassembly properties.

      You can also revisit your daylight subassembly settings to give the program a narrower offset or steeper slope or grade. Alternatively, you can adjust your alignment and/or profile to require less cut or fill, which would cause daylighting to occur over a shorter distance. Lastly, you can adjust your station range on the Parameters tab in Corridor Properties, as you did in the previous exercise.

      If this is not possible, omit daylighting through those specific stations, and once your corridor is built, do hand-grading using feature lines or grading objects. You can also investigate other subassemblies such as LinkOffsetAndElevation that will meet your design intention without requiring a surface target.

Creating a Surface from Link Data

Most of the time, you will build your corridor surface from links. As discussed earlier, each link in a subassembly is coded with a name such as Top, Pave, Datum, and so on. Choosing to build a surface from Top links will create a surface that triangulates between the points at the link vertices that represent the final finished grade.

The most commonly built link-based surfaces are Top for contours and Datum for earthworks; however, you can build a surface from any link code in your corridor. Figure 9.43 shows a schematic of how links are used to form the most common surfaces—in this case a top surface, as shown in the top image of Figure 9.43, and a datum surface, as shown in the bottom image of Figure 9.43.

When building a surface from links, you have the option of checking a box in the Add As Breakline column. Checking this box will add the actual link lines themselves as additional breaklines to the surface. In most cases, especially in intersection design, checking this box forces better triangulation.

Creating a Surface from Feature Lines

There might be cases where you would like to build a simple surface from your corridor—for example, by using just the crown and edge-of-travel way. If you build a surface from feature lines only or a combination of links and feature lines, you have more control over what Civil 3D uses as breaklines for the surface.

If you added all the topmost corridor feature lines to your surface item and built a surface, you would get a result that's similar to the result you would get if you had added the Top link codes.

Creating a Surface from Both Link Data and Feature Lines

A link-based surface can be improved by the addition of feature lines. A link-based surface does not automatically include the corridor feature lines but instead uses the link vertex points to create triangulation. Therefore, the addition of feature lines ensures that triangulation occurs where desired along ridges and valleys. This is especially important for intersection design, curves, and other corridor surfaces where triangulation around tight corners is critical. Figure 9.44 shows the Surfaces tab of the Corridor Properties dialog where a Top link surface will be improved by the addition of Back_Curb, ETW, and Top_Curb feature lines.

If you are having trouble with triangulation or contours not behaving as expected, experiment with adding a few feature lines to your corridor surface definition.

Creating a Corridor Surface for Each Link

To the right of the Create Corridor Surface button is the Create A Corridor Surface For Each Link button. Clicking this button populates the Surface List area with a multitude of corridor surfaces; as the button name would suggest, you will now have a corridor surface for each of the link codes.

This may not be desirable for many people because you rarely need a surface for every link, but once they are created, you can always remove the unwanted corridor surfaces using the Delete Surface Item button.

Using the Corridor Surface Name Template

The third button at the upper left of the Surfaces tab of the Corridor Properties dialog is the Surface Name Template button. When you click this button, the Name Template dialog shown in Figure 9.45 appears.

Here you can set the formatting for the corridor name as well as the number style, starting number, and increment value. The Property fields are Corridor Name and Next Corridor Surface Counter. Based on the name shown in Figure 9.45, the next surface created for the corridor named North River Crossing will be named North River Crossing Surface – (1). The Number style can be set to 1, 2, 3 … or 01, 02, 03 … or a multitude of other styles based on the number of leading zeros.

Completing Other Surface Tasks

You can do several other tasks on the Surfaces tab. For each corridor surface, you can set a surface style, revise the default name assigned by the Name Template, and provide a description for your corridor surface. Alternatively, you can do all those things once the corridor surface appears under the Surface branch in Prospector.

Boundary Types

There are several tools to assist you in corridor surface boundary creation. They can be automatic, semiautomatic, or manual in nature, depending on the complexity of the corridor.

You access these options on the Boundaries tab of the Corridor Properties dialog by right-clicking the name of your surface item, as shown in Figure 9.47.

The following corridor boundary methods are listed in order of desirability. Corridor Extents As Outer Boundary is the most user-friendly, whereas Add From Polygon is fast but needs constant updating because it is not dynamically linked to the corridor.

1. Corridor Extents As Outer Boundary With this selection, Civil 3D will shrink-wrap the corridor, taking into account intersections and various daylight options on different alignments. Corridor Extents As Outer Boundary will probably be your most-used boundary option unless you are modeling other parts of your roadway network in separate corridors. This topic will be covered in more detail in Chapter 10.
2. Add Automatically The Add Automatically boundary tool allows you to pick a point code and use the associated feature lines as your corridor boundary. This tool is available only for single-baseline corridors. This tool is automatic, is easy to apply, and will remain dynamically linked to the corridor.
3. Add Interactively The Add Interactively boundary tool allows you to work your way around a corridor and choose which corridor feature lines you would like to use as part of the boundary definition.

   Choosing this option is better than using Add From Polygon if Add Automatically and Corridor Extents are not available. This method is also good for defining hide boundaries in your corridor surface (hide boundaries were discussed in Chapter 4, “Surfaces”). It takes a bit of patience to trace the corridor, but the result is a dynamically linked boundary that changes when the corridor changes. Using this method, once you select a feature line, a thick line will trace around the corridor following your mouse; to switch to a different feature line, simply click the new feature line at the transition location. When it's complete, you can close the boundary just as you would a polyline.

4. Add From Polygon The Add From Polygon tool allows you to choose a closed 2D or 3D polyline or polygon in your drawing that you would like to add as a boundary for your corridor surface. This method is quick, but unlike the other methods, the resulting boundary is not dynamic to your design.

The next exercise leads you through creating a corridor surface with a shrink-wrap and an interactive boundary:

1. Open the 0905_CorridorBoundary.dwg or 0905_CorridorBoundary_METRIC.dwg file.
2. Select the corridor to activate the Corridor contextual tab.
3. From the Corridor contextual tab Modify Corridor panel, choose the Corridor Properties icon.
4. On the Surfaces tab of the Corridor Properties dialog, click the Create A Corridor Surface button in the upper-left corner of the dialog.
   

   You should now have a surface item in the bottom half of the dialog.

5. Click the surface item under the Name column and change the default name of your surface to North River Crossing - Top.
6. Verify that Links has been selected from the drop-down list in the Data Type selection box.
7. Verify that Top has been selected from the drop-down list in the Specify Code selection box.
8. Click the Add Surface Item button to add Top Links to the Surface Definition, set the Overhang Correction to Top Links, and fill in the Add As Breakline check box.
   
9. Click OK to accept the settings in this dialog. Choose Rebuild The Corridor when prompted and examine your surface.

   The road surface should look fine; however, because you have not yet added a boundary to this surface, undesirable triangulation is occurring outside your corridor area and the area bounded by ROAD A, B, C, and D.

10. Expand the Surfaces branch in Prospector.

    Note that you now have a corridor surface listed in addition to the existing surface that was already in the drawing.

11. Select the corridor again to activate the Corridor contextual tab if not already selected.
12. From the Corridor contextual tab Modify Corridor panel, choose Corridor Properties icon.

    If you do not see the Corridor Properties button on the Modify Corridor panel, you may have inadvertently chosen the corridor surface and may be viewing the Surface contextual ribbon.

13. On the Boundaries tab of the Corridor Properties dialog, right-click North River Crossing – Top in the listing.
14. Select Corridor Extents As Outer Boundary, which will define the outer boundary of the surface.
15. Click OK to accept the settings in the Corridor Properties dialog, and choose Rebuild The Corridor when prompted.
16. Examine your surface, and note that the triangulation terminates at the outer limits of the corridor.
17. Select the corridor again to activate the Corridor contextual tab if not still selected.
18. From the Corridor contextual tab Modify Corridor panel, choose the Corridor Properties icon.
19. On the Boundaries tab of the Corridor Properties dialog, right-click North River Crossing – Top in the listing.
20. Select Add Interactively.

    The Corridor Properties dialog will temporarily disappear while you define this boundary. You will now digitize around the area enclosed by ROAD A, B, C, and D, tracing over the edge of the corridor defining the enclosed area. You may find it helpful to turn on your endpoint Osnap setting, but it isn't necessary.

21. Using your scroll button, zoom into the northwest corner of the intersection of ROAD A and ROAD B.

    Be careful to avoid exiting the command while doing so.

22. Pick the edge of the corridor. If you see the Select A Feature Line dialog open, select Edge and click OK.
23. Drag the cursor to the left along the edge of the corridor until you see a red, dashed line representing your interactive boundary beginning to trace over the Edge feature line, as shown in Figure 9.48.

    

24. Pan to the left along the edge of the corridor until the red, dashed line makes a stop.

    It will make a stop when it reaches another baseline.

25. Click the stopping point and then click the link ahead of the stopping point to continue the generation of the interactive boundary.
26. Repeat step 25 whenever the interactive boundary makes a stop until you reach the point of closing your boundary.

    If you accidentally click the wrong location, just type U for undo to redefine the last segment. If the interactive boundary doesn't want to follow the edge of your corridor in certain areas, click the link the edge at the location just before the bad behavior begins. If the Select A Feature Line dialog opens displaying two Edge feature lines, just pick the top one and click OK.

27. To close your boundary, click the link just before the start point and type C for close.

    The Corridor Properties dialog will reappear.

28. On the Boundary tab of the Corridor Properties dialog, there are two boundaries listed.
29. Select Corridor Boundary(2) and click in the Use Type column to change the setting to Hide Boundary, as shown in Figure 9.49.

    

30. Click OK to dismiss the Corridor Properties dialog. Rebuild the corridor if prompted.
31. (Optional) Experiment with making changes to your finished grade profile, assembly, or alignment geometry and rebuilding both your corridor and finished ground surface to see the boundary in action.

When this exercise is complete, you can close the drawing. A finished copy of this drawing is available from the book's web page with the filename 0905_CorridorBoundary_FINISHED.dwg or 0905_CorridorBoundary_METRIC_FINISHED.dwg.

Common Surface-Creation Problems

Here are some common problems you may encounter when creating surfaces:

1. Problem Your corridor surface does not appear or seems to be empty.
   1. Typical Cause You might have created the surface item but not added any data. Another cause could be the surface style is set to No Display or to display contours at a wide interval, or the surface was created on a frozen layer.
   2. Fix Open the Corridor Properties dialog and switch to the Surfaces tab. Select a data type from the drop-down menus in the Data Type and Specify Code selection boxes, and click the Add Surface Item button. Make sure your dialog shows both a surface item and a data type, as shown in Figure 9.50.

      

2. Problem Your corridor surface does not seem to respect its boundary after a change to the assembly or surface-building data type (in other words, you switched from link data to feature lines).
   1. Typical Cause Automatic and interactive boundary definitions are dependent on the codes used in your corridor. If you remove or change the codes used in your corridor, the boundary needs to be redefined.
   2. FixOpen the Corridor Properties dialog and switch to the Boundaries tab. Remove any boundary definitions that are no longer valid (if any) by right-clicking the boundary and selecting Remove Boundary. Once the outdated boundary has been removed, you can redefine the corridor surface boundaries using any of the applicable boundary types.
3. Problem Your corridor surface seems to have gaps at points of curvature (PCs) and points of tangency (PTs) near curb returns.
   1. Typical Cause You may have encountered an error in rounding station values at these locations and as a result created gaps in your corridor. This is commonly caused by osnapping to start and end region stations using two-dimensional linework as a guide when some segments of that linework do not touch.
   2. Fix Be sure your corridor region definitions produce no gaps. You might consider using the PEDIT command to join lines and curves representing corridor elements that will need to be modeled later. You might also consider setting a COGO point at these locations (PCs, PTs, and so on) and using the Node object snap instead of the Endpoint object snap to select the same location each time you are required to do so.